Active Tectonics Impact on Society (1986) / Chapter Skim
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7. Tectonic Geomorphology of Escarpments and Mountain Fronts
7. Tectonic Geomorphology of Escarpments and Mountain Fronts
Pages 125-135
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From page 125... ...
Studies of p~leoseismicity, which is past seismicity based 12S on interpretation of smalbscale geomorphic or stratE graphic features, enable scientists to determine the past behavior of seismogenic structures and, hence, better evaluate the risk to society resulting from future earthquake~ The length of time ~ landform records depends on its survival bme, or how rapidly the landform evolves. Into types of landform that have long survival times are escarpments and mountain fronts.
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From page 126... ...
The circlein-square on the mountain fronts graph outlines the time period shown on the fault scarp graph. Likewise, the circle on the fault scarps graph indicates the time period covered by the historical earthquakes graph.
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From page 127... ...
, drainage basins fill all available space. For a + given mountain range, larger watersheds that tend to have a characteristic shape also have a regular spacing along the mountain front.
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From page 128... ...
LARRY MAYER deeply embayed by pediments have high sinuosity and may represent tectonic quiescence or rock control. A maximum limit on sinuosity is achieved where the spur ridges separating adjacent drainage basins are not eroded away.
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From page 129... ...
Second, when the mode of erosion of the valleyside slopes is dominated by slope retreat, then morphology of the stream valley is related to both the rate of stream downcutting and the rate of slope retreat. This concept is illustrated on Figure 7.6 and simply states that the volume of material removed when a stream valley is formed is related to both stream and hillslope erosional processes and that the resultant valley depends on the relative rates of the two processes.
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From page 130... ...
This is due in part to their antiquity, but also because the main drainage divide did not coincide with the escarpment. Significant drainages cut the Grand Wash Cliffs, the largest of which is the Colorado River.
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From page 131... ...
RATES OF GEOMORPHIC PROCESSES Rates of stream downcutting, escarpment retreat, Sedimentation, and other geomorphic processes permit estimates of the rates of tectonic processes such as vertical uplift. Pediments, for example, are bedrock surfaces formed by erosion of a mountain block that gently slope toward the basin and commonly are covered by some
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From page 132... ...
North is at top of photo. The north-south trending upper Grand Mash ChfE shows deep embayment by Creams traversing.
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From page 133... ...
Basalt flows and volcanic ash deposits provide an unique opportunity to determine the ages of tectonic events and the rates of geomorphic processes. They can preserve a datum that records previous river levels or a prefaulting topography.
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From page 134... ...
Field relations using dated basalts have supported tectonic geomorphologic inference regarding classical geomorphic problems including the development of the western margin of the Colorado Plateau and the evolution of the Grand Canyon (McKee and McKee, 1972; lIamblin et al., 1981~. DISCUSSION The tectonic geomorphology of fault-generated topographic fronts can be used to describe long-term tectonic FIGURE 7.15 Aerial oblique view looking north at Grand Wash.
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From page 135... ...
Rates of geomorphic processes such as pedimentation or stream downcutting can act as a clock that starts ticking following the formation of a tectonic landform. Present limited knowledge of these rates suggests that during periods of active faulting in the Basin and Range province, fault-slip rates of about 0.1-1 m per 10,000 yr or greater are needed to generate high topographic escarpments, and these periods of active faulting may last on the order of a million years.
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